For the exact weight determination of material streams as it is required for many loose materials, it is customary to successively fill, weigh and again empty a weighing vessel. This method has the disadvantage that the continuous material stream is continuously interrupted and that in addition it is mechanically costly.
For the continuous weight determination of material streams, in practice only conveyor belt scales are used by means of which the weight of a conveyor belt charged with loose material is continuously measured. In many processes this method is not usable because on the one hand it is fairly expensive and on the other hand it is very difficult to design it in a dust-proof manner.
A known method for the continuous volume measurement of pourable solids consists in dropping the delivered material from a predetermined height onto a bouncing plate and to measure the force of blow on the bounce plate. Provided that uniform conditions can be maintained, the force of blow is proportional to the conveying strength. It is not possible to maintain the measuring conditions such as the height of the drop, the friction between the loose material and the bouncing plate so uniform that sufficient accuracy can be obtained. Particularly in the lower measuring range, measuring errors occur due to air turbulence which make this method unusable for higher requirements in regard to the measuring accuracy, as they apply to the determination of weight.
For the determination of the mass or the volume of pneumatically supplied loose material in the iron industry, such as lime dust or fine ore, according to German Laid-Open application DE-OS No. 2,544,976 the effective inertial force (Coriolis force), which is produced in coupling a moving mass particle with a rotating reference body is used for the mass determination. In this method the material stream is made to impinge in axial parallel relationship on practically the entire surface of a disk provided with radial guide vanes, whereupon the loose material is flung off substantially perpendicularly to the rotational axis of the disk. The torque of the disk rotating at a constant rotational velocity varies in accordance with the Coriolis force, the variation of the torque being roughly proportional to the mass of the flowing material to be measured. With this variation of the torque the power consumption of the electro motor driving the disk varies; for this reason the current consumption of the electro motor can be measured and be used as a measure for the determination of the mass stream. For application in the iron industry for the determination of the charge volume the device described in DE-OS No. 2,544,976 may be sufficient, however for the exact determination of the flow of a pure loose material stream as for example granular food, this device is too inexact. The supply of the loose material perpendicularly to practically the entire guide vane surface of the rotating disk leads to uncontrollable bouncing off effects upon contact with the guide vanes so that the measuring accuracy is adversely affected. Depending on the location on the disk radius at which the mass particles impinge, the acceleration distance through the guide vanes is of a different length; different friction effects between the mass particles and the guide vanes also enter the measuring result. In addition the power consumption measurement of the electro motor is relatively inaccurate, because of its non-linear efficiency characteristic and the varying bearing friction.